A seismic instrumented cable (streamer) is an elongate cable like structure (often up to several thousand meters long), which comprises an array of hydrophone cables and associated with electric equipment along its length, and which is used in marine seismic surveying. In order to perform a 3D/4D marine seismic survey, a plurality of such instrumented cables is towed behind a seismic survey vessel. Acoustic signals produced by the seismic sources are directed down through the water and into the seabed beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones, and next digitized and processed to build up a representation of the earth strata in the area being surveyed.
The instrumented cables are typically towed at a constant depth of about five to ten meters, in order to facilitate the removal of undesired “false” reflections from the water surface. In order to keep the instrumented cables at a constant depth, control devices known as “birds” are attached to each instrumented cable at intervals of 200 to 300 meters.
Low frequency depth variations and lateral motions are inevitable. The main reasons for instrument cable depth variations are long periodic waves. In general, the worst-case situation is when towing in the same direction as the swell. Instrument cable lateral motions are mainly due to sea current components perpendicular to the towing direction. In the case of both swell and cross-current influences, the risk of streamer entanglement is therefore increased.
The instrument cable tension decreases proportional to the distance from the towing point. Therefore, low frequency instrument cable lateral and vertical motion tends to have larger amplitudes closer to the tail. However, the forces acting perpendicular to the instrument cable are non-uniformly distributed over the instrument cable length, and change over time as the towed array moves forward.
During a seismic survey, the instrument cables are intended to remain straight, parallel to each other, equally spaced and at the same depth. However, after deploying the instrument cables, it is typically necessary for the vessel to cruise in a straight line for at least three instrument cable lengths before the instrument cable distribution approximates this ideal arrangement and the survey can begin. This increases the time taken to carry out the survey, and therefore increases the cost of the survey. However, because of sea currents, the instrument cables frequently fail to accurately follow the path of the seismic survey vessel, sometimes deviating from this path at an angle, known as the feathering angle. This can adversely affect the coverage of the survey, frequently requiring that certain parts of the survey be repeated. In really bad circumstances, the instrument cables can actually become entangled, especially at the tail of the instrument cables, which can cause great damage and considerable financial loss.
U.S. Pat. No. 6,011,752 (Loran, D. Ambs et al.) describes a seismic instrumented cable position control module having a body with a first end and a second end and a bore therethrough from the first end to the second end for receiving a seismic streamer therethrough, at least one control surface, the at least one recess in which is initially disposed the at least one control surface movably connected to the body for movement from and into the at least one recess and for movement, when extended from the body, for attitude adjustment.
U.S. Pat. No. 6,671,223 B2 (Bittleston, Simon Hastings) describes a control device or “bird” for controlling the position of a marine seismic instrumented cable, which is provided with an elongate, partly flexible body which is designed to be electrically and mechanically connected in series with a instrumented cable. In a preferred form the control device has two opposed wings which are independently controllable in order to control the instrument cable's lateral position, as well as depth.
Moreover, it is known to use methods for controlling instrumented cables (streamers) which comprise the use of devices, such as GPS receivers, magnetic compass, acoustic transmitters, traditional hydrophones, or acoustic receivers, particularly dedicated for position determination. Use of externally mounted compasses has the disadvantage that they sometimes are lost or damaged due to the cables become tangled or in connection with other collision situations, and the compasses produce flow noise for adjacent seismic instrumented cables. In addition the compasses are powered by batteries which have to be replaced at certain intervals, and the compasses must be calibrated, repaired and replaced, which result in increased costs and time consumption.
From U.S. Pat. No. 5,761,153 it is known the use of both magnetic compasses and acoustic transmitter/receiver units, but these are also located externally, which make them vulnerable to damage as mentioned above in connection with use of compasses.
From U.S. Pat. No. 4,992,990 it is known the use of acoustic transmitter/receiver units arranged along the whole instrumented cable. The position is determined by trilateration of transmission times (and hence distance) between transmitter/receiver elements to form a triangle network, where two known positions are being used, preferably the position of a vessel and a float, whereas the transmitter/receiver unit is the third position being calculated in the triangle network. This method results in problems if mechanical or electrical failure occurs in hydrophone cables or other locations in the system. This publication also has the same problems as described above in relation to externally mounted transmitter/receiver units.
From U.S. Pat. No. 4,912,682 it is known the use of ultrasonic sonar transmitters which are positioned along an instrumented cable, and seismic receivers which are positioned along the instrumented cable, so that there are three receivers for each transmitter. However, this publication does not solve the disadvantages mentioned for the publications above.
U.S. Pat. No. 6,839,302 describe a solution of the problems mentioned above by suggesting a special section which can be mounted between traditional sections of the instrumented cable. However, this is an expensive and demanding solution and may in addition result in limited data redundancy and quality as it limits where the transmitter/receiver units can be located.
From U.S. Pat. No. 7,376,045 describes a system intended to solve all problems with the solution mentioned above. U.S. Pat. No. 7,376,045 describe a system comprising a number of acoustic transmitters mounted within the instrumented cables and arranged to transmit broadband signals having low cross-correlation between the signals from different transmitters; a number of acoustic receivers mounted within the instrumented cables and arranged to receive the signals from the transmitters; at least one processor arranged to cross-correlating the signals received by the receivers, having copies of the transmitter signals to determine transmitter identities of the received signals and to determine propagation times for the received signals; and a main processor arranged to transform the propagation times to distances between the identified transmitters and receivers, and to determine relative positions of the instrumented cables based on the distances. A substantial disadvantage of U.S. Pat. No. 7,376,045 is that it requires arrangement of transmitter/receiver elements in the instrumented cable which is space demanding in an instrumented cable. Another disadvantage is that the entire cable sections must be replaced in case of mechanical or electrical failure in the transmitter/receiver elements. In addition the distance from the point where the position is calculated will be different from the position where the control device is arranged, something which can result in inaccurate controlling of the instrumented cable.
Moreover, the control devices and control methods mentioned above are attended with numerous further disadvantages. Control devices hanging under the instrumented cable produces substantial noise as they are towed through the water. This noise interferes with the reflected signals detected by the hydrophone cables in the instrumented cables. Some of the control devices comprise a pair of wings or rudders which are mounted to a rotatable structure enclosing the seismic instrumented cable to generate a lift force in a specific direction. This is a relatively expensive and relatively complex electro mechanic construction which is very vulnerable in subsea operations.